Method of making ultraviolet transmitting high silica glass



Patented Apr. 25,, 1950 MnwnoaoaMaKmc ULTRAVIOLET amass- .JMITTJNGFH-IGH SILICA GLASS 7 ='ltif artin 'Eimery N1;i'iilb'erg, Corning, ""N. Y.,-assignor te ms-hing iii lasis Works-flowing, N.Y.,-a corapora ti-onmf-Newwork momm Application March 28, 1946, ".SBfiaTNO. 657,916

This invention "relates to "glasses which are :substantially transparent.torliltraviolet-radiations and has for its primary object -:to :provide a method for the sman-ufacture :of glasses which have .a .very low coefficient of .thermal expansion .and'wliich .are substantiallytransparent .to wave .ilen'gthsgdoum'to'2'54"millimrcrons (m;i).

Another objectis'toincreasethe transmission nf iultravi'ol'et radiationsin 'glass'eywhich contain Another object is to eliminate gases from such glasses. 1'

Another object is to provide a method for reducing polyvalent elements in such glasses to a low state of oxidation and to eliminate dissolved gases therefrom.

It is known that the transmission of short wave ultraviolet radiations through glass is greatly decreased or entirely prevented by 1 minute amounts of certain polyvalent or coloring oxides, such as F8203, T102, AszOs, etc., which may occur as impurities in the glass composition. It is also known that the substantial elimination of such oxides from the lass by using batch materials of a high degree of purity greatly enhances its transmission for short wave lengths. Reduction of the ultraviolet absorbent F6203 to the less absorbent FeO by melting the glass batch under reducing conditions or by the addition of a reducing agent or agents to the batch is also known as a further means of increasing the ultraviolet transmission.

In Patent 2,106,744 there is disclosed a method of producing a high silica glass by melting and fabricating an easily meltable glass of lower silica content and extracting therefrom soluble constituents other than silica by leachin to leave a practically pure silica skeleton with intercommunicating submicroscopic pores which can be consolidated to a non-porous condition by heating it at a temperature below its softening point.

As a result of the leaching pr0cess .,the amount of ultraviolet absorbing impurities remaining in the glass is less than thatwhichnormjally is found in ordinary glasses and its transmission'ffor ultraviolet radiations therefore is better than that of ordinary glasses made from the same materials. However, its ultraviolet transmission" isnot high 3 Claims. (cras -ea) enough for purposes requiring high transmission of wave lengths shorter than 300 m because a minute trace of F6203 persists in th'final glass.

On account of its high softening temperature and low expansion, tubes made of the above described glass should be particularly applicable for use as envelopes for medium pressure mercury =arc "lamps, butsuch use is attende'd'by unusual difiiculties. In the fabrication of the tubesinto *theenvelopes for such lamps, considerable flameworking of the glass tubes -"is necessary during "whiehpb j ectionable bubbling of the flame-worked "portions of the glass often occurs. The usual pre- Iim'inary baking out of the -envelope to elim inate'gas from its interior is'-not-eritirel-y-:efiective and a continuous, small but objectionable -evolu 'tionof gas from the interior of theenvelopeoccurs duringpperation'oftheJamp, whereby its efiective life is shortened. Moreover, arsenic, which is preferably used as a fining agent in melting the original glass, remains in the final glass in a small amount, which augments the tendency of the glass to bubble when flame-worked.

I have discovered a simple and inexpensive method whereby the above mentioned difliculties may be avoided and the objects of the invention attained. The new method consists in heating the porous silica skeleton, that is, tubes composed thereof, to a temperature insufficient to fuse it but suflicient to consolidate it to a non-porous condition while at the same time decreasing the partial pressure of oxygen within the pores to a very low value until consolidation is complete.

A low partial pressure of oxygen within the pores of the silica skeleton may be obtained by conducting the heating in a substantial vacuum. For this purpose an electrically heated vacuum furnace of the resistor type may be employed. Preferably, evacuation of the furnace is preceded by a preliminary heating of the article in an atmosphere of a reducin gas, preferably containing hydrogen, such as the non-explosive gaseous mixture known as forming gas which is composed of 92% nitrogen and 8% hydrogen.

In carrying out the invention the preformed tube or other article comprising a porous silica skeleton produced in accordance with the method described in the above mentioned patent is slowly heated up to about GOO-900 C. to dehydrate it. Heating should be sufiiciently slow to avoid rupture which might be caused by a sudden evolution of water vapor due to too rapid heating. The furnace is then evacuated and the temperature is increased to about 1225 C. by which time consolidation to a non-porous condition is usually complete. Preferably the temperature is held at about 1000 C. for about a half hour while under vacuum and before raising it to its maximum temperature.

Alternatively, the reducing gas may be introduced into the furnace at atmospheric pressure during the preliminary heating of the article and the air within the furnace displaced thereby. The introduction of the reducing gas into the furnace is of course stopped when the furnace is evacuated.

As a result of heating the porous glass article in a vacuum, the equilibrium which normally exists between F8203 and FeO is disturbed by the lack of oxygen and there is a tendency for oxygen to be abstracted from the F6203 until the latter is substantially converted to FeO, Also, by

heating in a vacuum reoxidation of FeO is pre- 1 vented. Best results are obtained if the article is preliminarily heated in an atmosphere ofreducing gas as described above. During the heating under vacuum, practically all of the arsenic which is present is evaporated into the vacuum and eliminated.

I claim:

a glass article of high silica content having throughout its mass a multiplicity of submicroscopic intercommunicating pores, heating the article in a reducing atmosphere, and thereafter heating the article in a substantial vacuum to a. temperatureinsuilicient to fuse it but sufiicient .to consolidate it to a non-porous condition until consolidation is complete.

- 3. In the process of manufacturing ultraviolet transmitting glass, the steps of forming a glass article'jofhigh silica content having throughout its mass a-multiplicity of submicroscopic inter- .communicatingpores, slowly heating the article in a, reduGing atmosphere from 600 to 900 C.

to dehydrate it without rupture, and thereafter heating the article in a substantial vacuum to if about 1225" C; until it is consolidated to a non- 1. In the process of manufacturing ultraviolet;

porous condition.

; MARTIN EMERY NORDBERG.

U REFERENCES CITED. l :The following references are of record in the file Tot-this patent: Um ED STATES PATENTS Number .Name Date 1,628,468 Miller May 10, 1927 1,779,175 Long Get. 21, 1930 2,215,039 Hood et a1. Sept. 17, 1940 2,355,746 Nordberg Aug. 15, .1944 

1. IN THE PROCESS OF MANUFACTURING ULTRAVIOLET TRANSMITTING GLASS, THE STEPS OF FORMING A GLASS ARTICLE OF HIGH SILICA CONTENT HAVING THROUGHOUT ITS MASS A MULTIPLICITY OF SUBMICROSCOPIC INTERCOMMUNICATING PORES, HEATING THE ARTICLE IN AN ATMOSPHERE OF A REDUCING GAS WHICH CONTAINS HYDROGEN, AND THEREAFTER HEATING THE ARTICLE IN A SUBSTANTIAL VACUUM TO A TEMPERATURE INSUFFICIENT TO FUSE IT BUT SUFFICIENT TO CONSOLIDATE IT TO A NON-POROUS CONDITION UNTIL CONSOLIDATION IS COMPLETE. 